1. Field of the Invention
This invention relates to differential temperature sensing, or detecting, devices and, more particularly, to improved, split-well differential temperature sensors, or detectors, for use in instrumentation systems for detecting the presence of water in a pressure vessel, such as a steam extraction pipe of a steam turbine system.
2. State of the Relevant Art
Differential temperature sensors, as are well known in the art, employ thermodynamic and fluid principles for selectively sensing the presence or absence of, and/or the creation or cessation of the flow of materials in a liquid or gaseous form. U.S. Pat. No. 3,366,942--Deane, illustrates one form of a prior art differential temperature sensor, used as a flow stoppage detector. The sensor, or probe, comprises a pair of heat sensing probes with a heater probe thermally connected therewith. The sensing and heater probes are adapted for being introduced into a conduit through which a material may flow. The heater probe is spaced more closely to one than to the other of the sensing probes. In the absence of flow, the sensing probe closer to the heater probe is at a higher temperature than the other sensing probe; conversely, when a fluid material flows past the probes, heat is conducted away from the heater probe and thus the temperature difference between the two sensing probes decreases, or disappears.
U.S. Pat. No. 3,898,638--Deane et al., illustrates another such differential temperature sensor, having the same basic configuration as that of the earlier Deane U.S. Pat. No. 3,366,942 but represented to have an improved internal structure of the temperature sensing probes which affords increased accuracy of measurements. As noted therein, differential heating of the two temperature sensing probes by the heater probe may be accomplished in part by, for example, the heat shunt running between the heater probe and the more adjacent of the two temperature sensing probes; further, both convection and/or conduction in the medium at rest, and conduction in the shunt, serve to carry heat differentially between the probes.
Another form of such differential temperature sensing probes, again having the same basic configuration of a pair of temperature sensors and a heater element disposed adjacent to one of the two temperature sensors, is disclosed in U.S. Pat. No. 4,449,403--McQueen. The particular application of the McQueen device entails utilizing plural such sensors in a vertically stacked array within a guide tube disposed within a reactor vessel, the outputs from the plurality of sensors providing an indication of the wet/dry condition of the coolant in the region of the fuel rods, among other purposes and functions. A particular concern in such reactor vessels is the presence of voids, e.g., a steam void, displacing the reactor coolant from the nuclear fuel rods, which then are inadequately cooled and may overheat. The composite device most specifically is disclosed for use in sensing the coolant properties under three regimes: subcooled (the normal operating condition); saturated liquid (the boiling condition); and saturated vapor (a voided condition). As noted therein, the improper conditions may result in "water hammer" effects producing pressure pulses which can break pipes, pipe supports, tanks, valves and other such vital equipment.
U.S. Pat. No. 4,440,717--Bevilacqua et al. likewise discloses an instrumentation system employing plural sensors at vertically spaced elevations and positioned within a nuclear reactor vessel, each sensor comprising a heater for heating one of a pair of thermocouples wired to provide both absolute temperatures and differential temperatures therebetween, for detecting the liquid coolant level within the vessel, again employing the difference in heat transfer characteristics between heat transfer to a liquid and heat transfer to a gas or vapor to sense the liquid level. Similar such sensors and related systems for use in nuclear reactor vessels or other pressurized water systems are disclosed in U.S. Pat. Nos. 4,418,035--Smith and 4,439,396--Rolstad. The Smith '035 patent moreover illustrates a block diagram form of a multiple function monitoring system employing such sensors.
While the differential temperature sensors, or detectors, of the present invention and instrumentation systems employing same as disclosed in the above cross-referenced applications have broad application, including use in sensing and monitoring pressure vessels of nuclear reactor systems as in the above-referenced patents, they have been developed with specific reference to the operation and preventive maintenance of steam turbine generators. Problems with such generators arising out of the induction of water or cool vapor into the steam turbines become more critical as the units age and particularly as they are used, increasingly, for cyclic and/or shift operation. Malfunctions of the equipment in the heat cycle can cause such induction to occur at various locations, including the main-steam inlet piping, the hot-reheat steam inlet piping, the cold-reheat steam piping, extraction connections, gland steam-sealing system, and turbine drains. Beyond the resulting structural damage and mechanical malfunctions caused by the induction of water or cool vapor, the resulting unscheduled down time of the equipment is a matter of serious concern.
In addition to the particular locations at which induction occurs, it is important to identify the various types of induction, i.e., the types of water induction events, which may occur. For example, induction may occur as a flow of a water film on the side of a pipe associated with the turbine produced typically by condensation of steam on the side of a cold pipe or from an overspray condition. Droplet or "chunk" flow may occur, visualized as a continuous projectile of water which may vary from the size of drops to walnuts and which may be mixed with steam. Slug flow may be produced, i.e., a slug of water which completely fills a section of pipe and is projected down the pipe, presumably by the flash-off of water. Two-phase flow as well has been identified, comprising generally an ill-defined "water-steam" mixture that may result from flash-off of high energy water, and may involve a core flow of solid water. Finally, a broad category exists wherein water may rise within a pipe, due to such sources as condensation, spray or flow, feed water heater tube leaks, and/or design deficiencies in the drain system, and to combinations thereof. It appears, however, that the vast majority of water induction events are of the slow rise type of the last category described, and which, moreover, may be the precursor to the other categories of water induction events. Thus, while not necessarily so limited in its scope, the sensors, or detectors of the present invention and the associated instrumentation systems are directed to this broader, last-mentioned category and thus to monitoring the condition within a pipe and more specifically for the detection of the relatively slow rise of water within a pipe associated with a turbine system. As noted, the sources of such water may be the boiler and feed water heaters, accumulation due to condensation, faulty sprayers and broken pipes, and accumulation arising from condensation within the turbine itself, in stages that operate in the wet region.
Beyond the specific sensors as disclosed in the foregoing patents, commercially available systems incorporating such differential temperature sensors for monitoring and detecting the presence of water have been developed. Solartron Protective Systems, a division of Solartron Transducers, owned by Schlumberger, offers a "Self-Validating Water Induction Monitoring System" under its registered trademark HYDRATECT - 2455D. Resistivity measurements are made inside of a manifold by means of electrodes, which serve to discriminate between the resistivities of water and steam (or air). As described in its sales literature, the energized tip of an electrode is referenced to the body of the manifold, and the tip is insulated from the body by a high purity insulator. Pairs of such electrodes may be mounted in two-port manifolds in conduits, such as drain lines, to be monitored, each electrode detecting the presence of either water or steam, and its output being routed by independent connections to an electronic discrimination circuit. A discriminator circuit purportedly checks for component failures and declares same as occurring, within each electrode channel. A validation check between two electrode channels subjected to the same conditions is described as being performed, as a basis for indicating whether a fault exists. The HYDRATECT - 2455D system of Solartron, however, is deficient in many respects and inherently incapable of providing reliable, long-life characteristics. For example, the sensor is of generally cylindrical configuration and is adapted to be inserted through a penetration in the sidewall of a pressure vessel and secured thereto, as is conventional. A segment of the cylindrical structure comprises an annular band of insulating material, which insulates the electrode tip of the sensor from the remainder of the structure. A tight pressure seal, e.g., a porcelain to metal weld, must be provided at the respective interfaces of the insulating band with the electrode and with the remainder of the cylindrical sidewall of the sensor. The interfaces of dissimilar materials, i.e., porcelain and metal, renders in the sensor structure highly susceptible to leakage and eventually breaking, particularly in view of the rather hostile environment to which it is subjected (e.g., temperature cycling, vibration and the like). In typical experience, such sensors have a reliable lifetime only of from one to three years, at most. Not only do sensors of this type fail to provide the long-life characteristics essential to an effective monitoring system, their tendency to leak and break presents a serious threat to personnel. Moreover, because of their structure, as described and as will be appreciated, the sensors cannot be repaired or replaced while the system, which they are intended to monitor, is on-line.
Another commercial system is offered by Fluid Components, Inc. and set forth in its brochure entitled "Liquid Level & Interface Controllers," that brochure citing protection for the disclosed systems under the above-referenced U.S. Pat. Nos. 3,366,942, 3,898,638 and 4,449,403. Sensors incorporating probes as disclosed in those patents are employed for measuring temperature differentials. The specific values of the output signals are stated to be governed by the media in contact with the probes and thus, for example, liquid/gas and liquid/liquid interfaces as well as wet/dry conditions purportedly may be detected. Monitoring and calibration circuits for the liquid level and interface controllers associated with the sensors are indicated to be available. These sensors and associated controllers, however, are not suitable for the hostile environment of steam turbine systems and, particularly, for performing the requisite sensing functions for anticipating problems of water induction. For example, the sensors cannot withstand the involved high pressure and temperature conditions. The sensors, moreover, are asymmetric and inherently lack any duplex functional capability as has been determined, in accordance with the present invention, to be essential to the effective and reliable monitoring and control of such systems. For example, an important fouling test, performed by the sensor and related system of the present invention, is incapable of being performed by an asymmetric sensor and a system incorporating same; moreover, since lacking any duplex configuration, there necessarily is no capability of on-line, automatic substitution for a failed element, e.g., a heater element. The specific structure of the sensors, moreover, does not permit physical replacement of failed heater and/or thermocouple elements while on-line. Moreover, such sensors and necessarily the related systems will not work in a steam flow environment in the absence of a shield surrounding the heater and thermocouple elements, since even low steam velocities will remove heat more rapidly than water.
Despite incorporating advances in technology, currently available sensors and monitoring and alarm systems employing same, as reported in the literature above-identified, have failed to satisfy critical needs in the industry. For example, the above-noted problem of water induction in steam turbines, while recognized and studied since the early 1970's, has yet to be adequately resolved.
Water induction incidents have become of such concern that the ASME (American Society of Mechanical Engineers) established a Committee on Turbine Water-Damage Prevention; plant design recommendations to prevent water damage are contained in ANSI/ASME Standard No. TDP-1-1985. More recently, studies done by the assignee of the present invention for EPRI in actual operating power generating facilities are set forth in a final report prepared and released by EPRI as report CS-4285, "Detection of Water Induction in Steam Turbines. Phase III: Field Demonstration." These studies emphasize the continuing, critical need for reliable sensors and monitoring systems for use in the environment of steam turbines, to detect the severe problem of water induction.